ls-remote: fix synopsis

git ls-remote takes an optional get-url argument, and specifying the
repository is optional.  Fix the synopsis in the documentation to
reflect this.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-ls-remote.txt

@@ -10,7 +10,7 @@ SYNOPSIS
 --------
 [verse]
 'git ls-remote' [--heads] [--tags] [--refs] [--upload-pack=<exec>]
-	      [-q | --quiet] [--exit-code] <repository> [<refs>...]
+	      [-q | --quiet] [--exit-code] [--get-url] [<repository> [<refs>...]]
 
 DESCRIPTION
 -----------